This invention relates to coupled cavity travelling wave tubes.
A travelling wave tube (TWT) is a device in which an RF (radio frequency) signal and electron beam are made to interact in such a way as to amplify the power of the RF signal. A coupled cavity TWT includes an elongate hollow tube, generally of circular or rectangular cross-section, having a plurality of walls arranged transverse to its longitudinal axis to divide its interior into a number of cavities. The centre of each wall has a passage therethrough, known as a drift tube, which is aligned with the longitudinal axis and through which the electron beam passes during operation of the TWT. The drift tubes are commonly extended in longitudinal length by tubular projections on one or both sides of their walls. Each wall also includes a slot which allows RF coupling between adjacent cavities.
Typically, the walls are designed to project beyond the part of the hollow tube which defines the lateral dimension of the cavities, giving a finned appearance. The walls in such a structure are commonly of iron, or some other ferro-magnetic material, and magnetic material is located between the projecting parts of the walls. A magnetic focusing field may thus be set up axially along the tube, tending to collimate the electron beam.
However, even when such magnetic focussing is employed, some electrons collide with the inner surfaces of the drift tubes. The energy of the electrons is dissipated into the iron causing its temperature to rise. If the temperature reaches more than about 400.degree. C. the magnetic permeability of the iron is reduced, and the magnetic field is reduced, increasing the tendency of the electrons to collide with the surfaces of the drift tubes. Since iron is a poor conductor of heat, this effectively limits the power at which such a TWT may operate.
In one method previously employed to overcome this limitation, the walls are made of laminated iron and copper, the copper layer being intended to provide a thermal path for energy dissipated in the iron. However, this introduces some complexity in the manufacture of the structure, and hence increases its cost. A more serious objection is that optimum operation of the TWT is achieved by, amongst other things, having a certain ratio for the distance between adjacent drift tubes and the thickness of the walls. Thus, if the copper layer is simply added to the iron, the wall thickness is increased, and this results in a reduction in the impedance of the structure, which is undesirable, since it reduces the power output. To overcome this objection it is therefore necessary to reduce the iron content of the wall by an amount comparable to the amount of copper added. However, this leads to a reduction in the magnetic saturation level and may impair the magnetic focussing effect.
Another proposed method is to coat the outside of the iron walls with a thin copper layer. However, this again reduces the impedance of the structure and also introduces a capacitance between the copper layers on facing adjacent walls, reducing the impedance still further and lowering the power output.